As Charles Lavin mentions in this post, and Miles Li [msft] provides solution in same post
By default the CM will route all traffic over the vpn, not just internal traffic. If you have a high latency or low bandwidth connection this could cause problems.
http://www.microsoft.com/communities/newsgroups/list/en-us/default.aspx?dg=microsoft.public.windows.server.sbs&tid=51ac74d3-f1e3-4430-bbfa-c062b37f1341&cat=&lang=&cr=&sloc=&p=1